HfGlENE OK THE NERVOUS SYSTEM 317 



which recuperation becomes doubtful, wholly impossible, 

 or only partial. 



463. Fatigue of Nerve Cells. A certain amount of activ- 

 ity in cells of all sorts is promotive of health. It causes 

 a more rapid flow of the blood and greater storage of 

 fresh matter to be afterward used in the liberation of 

 energy. But too prolonged exercise or stimulation leads 

 to exhaustion of the cells, and for every one there is a limit 

 to the power of restoration after exercise. In the last 

 stages of extreme fatigue it is the nerve cells and not the 

 muscles which succumb to exhaustion. Microscopic exam- 

 ination of animals shows that the cells of the nerve centers 

 gradually become shrunken and irregular in outline under 

 stimulation, while the nuclei within the cells and in the 

 inclosing wall are also seen to diminish. After refreshing 

 sleep the cells are at their full size, the blood current passes 

 at the normal rate through the nerve centers, and they 

 readily respond to stimulus. At the close of the day a 

 marked change has occurred in the cells of those centers. 

 The rate of blood circulation has declined, and the stream 

 is loaded with the accumulated products of the day's activ- 

 ities, while the diminished size of the nerve cells and their 

 nuclei proclaims a state of fatigue. 



464. Sleep. It is by means of sleep that the nerve cells 

 regain their form and their readiness to react on receiving 

 stimulus. During sleep the nervous activities are greatly 

 diminished. Afferent impulses do not set in motion the 

 complex cerebral processes of waking hours. Conscious- 

 ness is not excited, and memory fails to register the recep- 

 tion of impulses. Reflex movements may indeed result, 

 but in the deepest sleep even such action is mainly lack- 

 ing. Respiration and the circulation of the blood con- 

 tinue, but become slower, and a large amount of blood is 



MACY'S PHYS. 20 



